System for controlling a steam turbine at start-up

ABSTRACT

A reheat turbine having a control valve in a reheat conduit to provide a system for switching from controlling the turbine utilizing a main stop valve control to controlling the turbine utilizing a sequentially operable governing valve so that there is a minimal change in the steam temperature in the first stage of the turbine as a result of switching from one mode of control to the other.

United States Patent 1191 Ishikawa June 4, 1974 [54] SYSTEM FOR CONTROLLING A STEAM 3.241.322 3/1966 Strohmeyer, Jr. 60/105 x TURBINE AT S -pup 3,286,466 11/1966 Stevens 60/105 3,338,053 8/1967 Gorzegno et al. 60/l05 [75] Inventor: Motos s aw Nagasakl, 3,577,733 5/1971 Manuel et al 60/105 Japan Assigneel Mifsubishi Jukogyo Kabllshiki Primary Examiner-Edgar W.Geoghegan Kalsha, Tokyo, Japan Assistant Examiner-Allen M. Ostrager '[22] Filed; Oct 3, 1972 Attorney, Agent, or Firm-F. J. Baehr, Jr.

[21] App]. No.: 294,763 r -[57] ABSTRACT F g g [30] g? Priority Data 46 8 44 A reheat turbine having a control valve in a reheat Oct. l, apan 28 Y conduitto provide a system fot Switching from com trolling the turbine utilizing a main stop valve control 2% 5 60/657 60/ to controlling the turbine utilizing a sequentially operd 657 679 able governing valve so that there is a minimal change 1 in the steam temperature inthe first stage of the tur-' I bine as a result of switching from one mode of control [56] References Cited to the other UNITED STATES PATENTS I 3,097,488 7/1963 Eggenberger et al. 60/105 X 8 Claims, 6 Drawing Figures SYSTEM FOR CONTROLLING A STEAM TURBINE AT START-UP BACKGROUND OF THE INVENTION This invention relates to a system for controlling a steam turbine and more particularly to a system for controlling a reheat turbine during start-up and at low loads.

Large reheat steam turbines have a plurality of inlet nozzle chambers disposed circumferentially adjacent a first stage of rotating blades, and during start-up and at low loads the quantity of motive steam being supplied to'the turbine is small so that motive steam is only flowing through one nozzle chamber. This results in nonuniform heating of the turbine, which causes thermal stress, which reduces the life expectancy of the turbine.

To provide uniform heating of the turbine, the sequentially operable governing valves are all opened allowing motive steam to be admitted to all portions of the first stage and the main steam throttle valve is operated to control the speed of the turbine. This method of operation is referred to as full arc admission and is utilized during start-up and when operating at low loads. However, during normal operation of the turbine it is desirable to control the turbine with the sequentially operable governing valves and when switching from one mode of operation to the other, thetemperature of the motive steam in the first stage of the turbine fluctuates, as each mode of operation produces different steam temperatures as the throttling characteristics and distribution of the steam varies considerably for each mode of operation.

SUMMARY OF THE INVENTION steam throttling valve in fluid communication with the I motive steam b'eing'supplied to the first turbine, and a sequentially operable control valve disposed in series with the main steam throttling valve. The main steam throttling valve and the sequentially operable governing valve are adapted to be independently operable to control the flow of motive steam through the first turbine to maintain a generally constant speed. This control system, also comprises a control valve for controlling the flow of motive steam flowing through the reheat conduit and a device responsive to the condition of the motive steam, after it passes through at least one portion of the first turbine, to control the control valve in the reheat conduit, whereby the temperature of the motive steam exhausted from the first turbine remains generally constant irrespective of whether the quantity of motive steam is controlled by the main steam throttling valve or by the sequentially operable control valve thereby minimizing the thermal shock resulting from switching from one mode of control to the other.

BRIEF DESCRIPTION OF THE DRAWINGS The objects and advantages of this invention will become more apparent from reading the following detailed description in connection with the accompanying drawings. in which:

FIG. 1 is a schematic diagram of nozzles chambers for a turbine having a plurality of sequentially operable control valves;

FIG. 2 is a graph showing the operation of the turbine with partial'and full part admission'su'perimposed upon a portion of a Mollier diagram;

FIG. 3 is a flow diagram for a reheat turbine;

FIG. 4 is a schematic diagram of a reheat turbine incorporating a control system made in accordance with this invention;

FIG. 5 is a schematic diagram of a conroller utilized in this invention; and a FIG. 6 is a temperature-time graph showing the advantages of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS a high pressure turbine 9. The exhaust .fromthe high. pressure turbine 9 flows through a reheat conduit 11 to I a reheater 13, wherein additional heat is added to the partially used motive steam to increase its temperature. The reheat conduit 11 directs the reheated motive steam through a stop valve 15 an intercept valve .17 and then into an intermediate pressure turbine 19. The exhaust from the intermediate pressure turbine 19 flows through'a crossover pipe 21 to adouble flow low pressure turbine 23. The exhaust from the low pressure turbine is condensed in a condenser 25. The condensate is picked up from a hot well by a condensate pump 27, pumped through two low pressure feedwater heaters 29 and 31, respectively, and then to a boiler feed pump 33 which pumps the condensate through two high pressure feedwater heaters 35and 37 and then into the boiler l forming a closedcycle. Turbines 9, l9 and 23 are disposed in a tandem compound arrangement and are directly connected by a common shaft 38 to a generator 39, which produceselectrical power and provides the load for the turbine. I

The stop valve 5 and the control valve 7 are adapted to independently respond to the speed and load so that either may be .utilized to control the flow of motive steam to the turbine. The stop valve 5 may be one or more valves, which are operable to provide motive steam throttling means, and the control valve 7 may be a sequentially operable control or governing valve meanscomprising a plurality of sequentially operable valves 7a, b, c, d, e, f, g and h disposed to regulate the flow of motive steam to a plurality of nozzle chambers 41a, b, c, d, e, f, g and h disposed circumferentially adjacent a first stage of rotatable blades 43, as shown in FIGS. 1 and 3.

As shown in FIG. 4 the control system for the turbine comprises the intercept valve 17 which has a servomotor 45 or other means for regulating the opening and closing thereof in order to regulate the flow of motive fluid as steam through the reheat conduit 11. The flow of motive steam through the reheater conduit 11 may also be controlled or regulated by a bypass valve 47, which has a servomotor 49 cooperatively associated therewith, or other bypass means which direct the motivestearn around the reheat stop valve 15 and the intercept valve 17.

, produce a signal, which may be either pneumatic, electric or hydraulic and sends it to a controller 55 or 57 which in turn sends a signal to the servomotor 45 or-49 to open and close the intercept valve 17 or the bypass valve 47, respectively, to regulate or control the quantity of motive steam flowing from the exhaust of the first or high pressure turbine 9 and through the reheater l3 and reheater conduit 11.

The controllers 55 and 57 may be pneumatic, electric or hydraulic, however, the ones shown in F IG. are hydraulic and comprise three compartments 61, 62 and 63. A bellows, diaphragm or other pressure sensitive or pressure responsive means 65 is disposed between the compartments 61 and 62. Cooperatively associated with the pressure-responsive means 65 is a valve stem 67 which has a valve plug.69 connected thereto. The valve plug 69'is cooperatively associated with an opening 71 between chambers 62 and 63 so as to regulate the flow of fluid'therebetween.

High pressure oil'is supplied to the compartment 62 through an orifice 73 and-is discharged from the compartment 62 via port 75 which is in fluid communication with the servomotor 45 or 49. The means responsive to a condition of the motive steam may be pressure or temperature responsive means 51 for 53, respectively, and the signal they produce is a varied pressure, which acts upon the pressure responsive means 65to open and close the valve plug 69,which when opened,

shunts a portion of the high pressure oil supplied to the compartment 62 to a drain port 77 disposed in compartment 63. Thus regulating the pressure of the oil supplied to the servomotors 45 or 49 to modulate the valve 17 or 47, respectively, and thereby regulates the flow of motive steam through and reheat conduit 11.

' Solenoid valves 81 and 83 are disposed in conduits between the controller 55 and 57 and the servomotors 45 and 49, respectively, so that the control system can .beremoved from service automatically or manually when the load exceeds some predetermined value.-

Since the intercept valve 17 performs other functions in the'control system, a solenoid valve 85 is provided to drain high pressure control fluid being supplied thereto to drain to cause the intercept valve to close rapidly. A solenoid'valve 87 is provided to shut off pressurized control fluid from other sources whenthe intercept valve is being utilized to regulate the flow of motor fluid through the reheat conduit 11.

To assist in understanding this invention, various steam conditions are represented on an enthalpyentropy graph or a portion of a Mollier chart.

P represents the pressure of .the steam in the main steam line and t represents its temperature.

P, I represents the pressure and temperature of the steam after it has passed .through the wide opened main steam throttling valve 5 and through the partially opened sequentially operable governing valve 7. Since no work has been done by the steam as it passes through the valves the Rankine cycle efficiency would be approximately zero and the enthalpy would be essentially equal to the enthalpy at the conditions P.

P, t, represents the pressure and temperature, re-

spectively, of the steam after it has passed through the wide open main steam throttle valve and the partially P t represents the pressure and temperature, respectively, of steam after it has passed through 'a partially opened main steam throttle valve 5 and a fully opened sequentially operable governing valve 7 assuming a Rankine Cycle Efficiency of zero, and P ti represents the condition of this steam after it has passed through the first stage 43 of the turbine 9 assuming a Rankine Efficiency of approximately 65 percent.

P 1 represents the condition of the steam after passing through the wide opened main steam throttling valve 5 and a wide opened sequentially operable governing valve-and through the first'stage 43 of the turbine 9 assuming generally 65 percent Rankine Efficiency of the first stage of the turbine.

Line tc represents a constant temperature line and lines Pc ,.Pc and Pe represent constant pressure lines.

Thus, indicating that by controlling the pressure after the first stage of the high pressure turbine utilizing the intercept or bypass valve, the temperature of the steam after it leaves the first stage can be adjusted so that there is no material temperature difference when switching from one mode of operation to the other.

FIG 6 shows the temperature of the steam after the first stage of the high pressure turbine as the function of time from start-up. The solid line 91 represents the temperature versus time curve for control systems previously used and the dotted line 93 represents the temperature time curve for control systems made in accordance with this invention.

What is claimed is:

l. A control system for controlling the flow of motive steam passing through a turbine unit having a first turbine, areheater, a second turbine and conduit means fluidly connecting saiditurbines and reheater, said con trol system comprising main stream-throttling means in fluid communication with the motive steam-being supplied to the first turbine, sequentially operable control means disposed in series with said main steam throttling means, said main steam throttling means and said sequentially operable control means each being inde pendently operable to control the flow of motive steam to the first turbine to maintain a generally constant speed, means for controlling the flow through said conduit means, and means responsive to a condition of the motive steam after it passes through at least a portion of said first turbine for controlling the means for controlling the flow through the conduit, .said means for controlling the flow through the conduit means cooperating with said steam'throttling means and said sequentially operable control means to maintain the temperature of the motive steam exhausted from the first turbine generally constant irrespective of whether the quantity of motivesteam passing through the first turbine is being controlled by the steam throttling means or by the sequentially operable control mea.ns,.thereby minimizing the thermal shock resulting from shifting from one mode of independentlyoperable control to the other. f I

2. 'A control system as set forth in claim '1, wherein the means responsive to a condition of the motive steam is responsive to the pressure of the motive steam.

3. A control system as set forth in claim 1, wherein Y the first turbine has a plurality of stages and the means responsive to a condition of the motive steam is responsive to the pressure of the motive steam as it leaves the first stage.

4. A control system as set forth in claim 1, wherein the means responsive to a condition of the motive opening and closing thereof.

7. A control system as set forth in claim 1, wherein the conduit has shut off valve means disposed therein and the means for regulating the flow through the conduit is bypass means adapted to shut the flow of motive fluid around said shut off valve means, said bypass means having control valve means disposed therein.

8. A control system as set forth in claim 1, wherein the conduit has a trip valve and an intercept valve disposed therein and the means for regulating the flow through the conduit is bypass means which shuts motive fluid around said trip and intercept valve, and said bypass means has control valve means disposed therein. 

1. A control system for controlling the flow of motive steam passing through a turbine unit having a first turbine, a reheater, a second turbine and conduit means fluidly connecting said turbines and reheater, said control system comprising main stream throttling means in fluid communication with the motive steam being supplied to the first turbine, sequentially operable control means disposed in series with said main steam throttling means, said main steam throttling means and said sequentially operable control means each being independently operable to control the flow of motive steam to the first turbine to maintain a generally constant speed, means for controlling the flow through said conduit means, and means responsive to a condition of the motive steam after it passes through at least a portion of said first turbine for controlling the means for controlling the flow through the conduit, said means for controlling the flow through the conduit means cooperating with said steam throttling means and said sequentially operable control means to maintain the temperature of the motive steam exhausted from the first turbine generally constant irrespective of whether the quantity of motive steam passing through the first turbine is being controlled by the steam throttling means or by the sequentially operable control means, thereby minimizing the thermal shock resulting from shifting from one mode of independently operable control to the other.
 2. A control system as set forth in claim 1, wherein the means responsive to a condition of the motive steam is responsive to the pressure of the motive steam.
 3. A control system as set forth in claim 1, wherein the first turbine has a plurality of stages and the means responsive to a condition of the motive steam is responsive to the pressure of the motive steam as it leaves the first stage.
 4. A control system as set forth in claim 1, wherein the means responsive to a condition of the motive steam is a temperature responsive means.
 5. A control system as set forth in claim 1, wherein the first turbine has a plurality of stages and the means responsive to the condition of the motive steam is a temperature responsive means.
 6. A control system as set forth in claim 1, wherein the means for controlling the flow through the conduit is an intercept valve having means for regulating the opening and closing thereof.
 7. A control system as set forth in claim 1, wherein the conduit has shut off valve means disposed therein and the means for regulating the flow through the conduit is bypass means adapted to shut the flow of motive fluid around said shut off valve means, said bypass means having control valve means disposed therein.
 8. A control system as set forth in claim 1, wherein the conduit has a trip valve and an intercept valve disposed therein and the means for regulating the flow through the conduit is bypass means which shuts motive fluid around said trip and intercept valve, and said bypass means has control valve means disposed therein. 